Recent advances in communications technology have created a burgeoning telephony market, both in terms of the availability of new services and new forms of equipment. Cellular telephone technology is one of those new services now being routinely offered in many locations across the country. While cellular telephony is a powerful and efficient new medium, it has also become an important tool for carrying out illegal activities. The mobility and perceived privacy of cellular mobile communications holds significant appeal to those engaged in illegal activities. In fact, in many areas drug dealers routinely use cellular telephony to conduct their day-to-day illegal drug deals.
A prior art cellular system 10 is illustrated in FIG. 1. The cellular system 10 consists of a frequency-modulation (FM) radio network covering a series of geographical areas referred to as cells and identified by reference character 12 in FIG. 1. The two-way radios in each mobile unit, commonly referred to as cellular telephones, operate within the cells 12 by communicating between the mobile unit 14 and a base station 16 within each cell 12. The cellular system 10 is defined by a plurality of base stations 16 distributed over a geographical area of system coverage and managed and controlled over links 17 by a centralized switch referred to as the mobile telephone switching office 18 (MTSO). The base station 16 within each cell 12 has the responsibility for controlling and communicating with all mobile units 14 within the cell and for relaying voice traffic between the mobile units 14 and the mobile telephone switching office 18. The MTSO 18 then relays the voice traffic to the public service telephone network 20 (PSTN) over a link 19.
There are four frequencies used between each base station 16 and any mobile unit 14. Two forward frequencies are used for communication from the base station 16 to the mobile unit 14 and two reverse frequencies are for communications from the mobile unit 14 to the base station 16. One forward frequency is paired with one reverse frequency for communicating control and status information between any mobile unit 14 and the base station 16. A second forward frequency is paired with a second reverse frequency for communicating voice and data between the base station 16 and the mobile unit 14. By convention, these various frequencies are referred to as the forward control channel, the forward voice channel, the reverse control channel, and the reverse voice channel, respectively. Each cell 12 has a single forward control channel and a single reverse control channel for all control and status messages between mobile units 14 within the cell and the base station 16 of the cell. Each cell 12 has a number of forward and reverse voice channels for assignment by the base station 16 as required to meet the communications traffic demands. The forward voice channel is for communicating voice and data from the base station 16 to the mobile unit 14, while the reverse voice channel is the frequency on which voice/data is passed from the mobile unit 14 to the base station 16.
The following is an example of the technique for setting up an external call in the cellular system 10. When the user of the mobile unit 14 first turns on his cellular telephone, the unit scans all forward control channels to determine the strongest one. Recall that each cell 12 has a base station 16 and each base station has a unique forward control channel. The mobile unit 14 then locks onto the strongest received forward control channel, presumably the forward control channel for the cell 12 in which the mobile unit 14 is located, and continues to monitor it.
When a call is placed to a mobile unit 14 from the conventional telephone network, a global page message (or MTSO page), which includes the telephone number or electronic serial number of the called mobile unit, is generated at the mobile telephone switching office 18. The global page is then transmitted in digital format to the base station 16 of every cell 12 that is within the mobile telephone switching office 18. This multiple transmission occurs because the mobile telephone switching office 18 does not know where the target mobile unit 14 is, or if it is within the range of the mobile telephone switching office 18. Each base station 16 then transmits the global page on its unique forward control channel. When the activated mobile unit 14 identifies its own telephone number within the global page, it responds to the base station 16 on the reverse control channel, basically saying, "Here I am". The called mobile unit 14 responds on the dedicated reverse control channel that is unique to the cell 12 in which it is located, and therefore none of the other base stations 16 will see a response to the global page. The base station 16 that is tuned to that reverse control channel sees the response and selects a forward voice channel/reverse voice channel assignment for the mobile unit 14. A command (referred to as a base station page) is then transmitted to the mobile unit 14 from the base station 16 in the form of a voice channel assignment, "I hear you, and please tune to channel x to execute your call". Upon receiving this message, the mobile unit 14 tunes its receiver to the designated forward voice channel to hear the called party and tunes to the paired reverse voice channel to transmit.
This technique for call set up is further complicated as the mobile unit 14 passes from one cell to another during the conversation. The controlling base station 16 continually monitors the strength of the signal from the mobile unit 14 and if that strength diminishes significantly (perhaps indicating that the mobile unit 14 has entered a different cell) then a cell hand-off occurs. To accomplish this, a data message is transmitted from the base station 16 on the forward voice channel telling the mobile unit 14 to tune to a different voice frequency, one that is controlled by a neighboring cell. The mobile unit 14 retunes and the conversation continues on the new frequency. Many cell hand-offs may occur during a single conversation as the mobile unit 14 exits and enters cells.
When the call is completed, the mobile unit 14 scans and locks onto the strongest forward control channel, which again indicates the cell 12 in which the mobile unit 14 resides and listens for a global page that contains its telephone number or electronic serial number.
As discussed above, the mobile unit 14 receives on the forward voice channel and transmits on a different frequency, the reverse voice channel. This would imply that one would have to monitor both frequencies to hear both sides of the conversation. Such is not the case, however. Because it is unnatural for the telephone user to speak into the telephone and not hear himself in the earpiece. The telephone company transmits the user's voice back for reproduction by the earpiece. This technique is used in the cellular telephony system so that both sides of a conversation can be heard by monitoring only the forward voice channel.
While legal wiretaps can be performed on cellular telephones, where the tap itself is located at the mobile telephone switching office, this type of wiretap is not a complete solution to the problem of monitoring these conversations. If the target mobile telephone is used outside the coverage area of the tapped MTSO coverage is lost. This problem is especially troublesome in areas that are located on the boundaries between adjacent MTSO's. It is obvious that a "fixed site" wiretap on a mobile telephone system cannot be very effective. Therefore, an approach using the mobile communications medium, namely radio frequency reception, has significant advantages. The present invention, as will be described further herein, uses such a radio frequency approach.
One prior technique for monitoring cellular telephone conversations is simply to tune a single scanner or receiver to a forward voice channel and listen to the cellular conversation on that frequency (See FIG. 2). Clearly, this is a hit and miss technique as it requires continuous scanning of the forward voice channels to find a specific target mobile unit. This simple approach provides no way of: (a) following the call during a hand-off; (b) knowing who is talking unless names are spoken; (c) knowing what phone number is being used, other than the fact that it is a cellular telephone; and (d) linking target telephone numbers with cellular conversations (i.e., finding the telephone conversations of the target mobile unit when you know the target's telephone number or electronic serial number). The tape recorder 30 must be controlled manually and the receiver 32 must be manually tuned. These limitations prevent effective use of this technique for legal wiretaps by law enforcement agencies.
A second prior art scheme (FIG. 3) provides the additional feature of following the cellular telephone conversation as it shifts from one voice channel frequency to another, as the target mobile unit passes from one cell to another. This embodiment increases the complexity of the listening device by the addition of the ability to receive and decode the digital data bursts that identify the new voice channel assignment occurring on the existing voice channel frequency. These digital data bursts tell the receiver which frequency to tune to next. The two additional functions required to accomplish this task are identified as the signal processing function 34 and the processing function 36 in FIG. 3. The signal processing function 34 converts the frequency shift keyed (FSK) tone bursts (analog) into digital data by performing a simple FSK demodulation. The digital data representing the new voice channel frequency is communicated to the processing function 36. Here it is decoded and the processing function 36 then commands the receiver 32 to tune to the new frequency. In this embodiment the processing function 36 is manually tuned to establish the initial voice channel frequency to which the receiver 32 is to be tuned. The disadvantage with this embodiment is the inability to find a mobile unit having a particular telephone number or electronic serial number. Also this technique does not provide the telephone numbers (called or calling) associated with any call that is being monitored. Another limitation of this technique is its ability to only monitor a single forward control channel.
The choice of which single forward control channel to monitor is critical to the probability of intercepting a particular target due to the unique control channel allocation scheme that is based on geographical location. Recall that each cell in a cellular telephone system operates at only one forward control channel frequency. If the mobile target is not located within the coverage area of the cell using the monitored forward control channel, then the voice channel assignment to that target unit will not be available to the intercept system. Therefore, the user of such an intercept system needs prior knowledge of the cell in which the target mobile unit is operating. In those few situations where the correct forward control channel is the one monitored and the voice channel assignment is therefore received, then the intercept unit retunes its receiver to the commanded forward voice channel. The intercepter can now monitor the data on the forward voice channel and is prepared for a cell hand-off when it occurs, as described above.
In yet another embodiment (See FIG. 4) more processing power is included in the processing function 36, providing the ability to store a telephone number that the user is looking for. In this embodiment additional receiver control is added so that the receiver can first be tuned to the forward control channel for receiving telephone numbers and electronic serial numbers. Once the target number is located, the receiver locks onto that target mobile unit, starts the tape recorder, and follows the telephone conversation, including cell hand-offs. This embodiment requires the entering of the target telephone number and then tuning the receiver 32 to the desired forward control channel. This target telephone number .information is input to the processing function 36. The processing function 36 also activates the tape recorder 30 when there is a match between the target telephone number and the telephone number picked up by the receiver 32.
If the target mobile unit was stationary or at least began its cellular communication in the same cell site every time, the FIG. 4 embodiment would be satisfactory. However, the nature of cellular communications is mobility. The user therefore does not know where a cellular conversation will originate. The best that can be hoped for is the general vicinity of where that conversation will begin. To solve this problem one could simply add many receivers, each having the functions previously discussed in conjunction with FIG. 4. Such a system would include the number of receivers necessary to cover the area that the target mobile unit might be in, with each receiver tuned to a different cellular control channel within that area.
With this extension of the FIG. 4 embodiment the system now has the capability to intercept cellular telephone calls for a fixed number of cell sites over a geographical area. The limitations of this system include: (a) each receiver must be manually tuned to a designated forward control channel and there must be some scheme for determining the forward control channel of choice; (b) if the target mobile unit has moved out of the geographical area, none of the conversations will be captured; (c) in this scheme the voice channel communications associated with a particular target are handled by only that receiver that was monitoring the forward control channel containing the voice channel assignment for the target mobile unit, and (d) since there is one tape recorder for every receiver, the tape recorder can only record those conversations that the receiver with which it is associated is monitoring. As a result of this last stated disadvantage, if the cellular intercept user is trying to gather evidence on a particular person or group of people, that evidence would be spread over several or all the tape recorders. For evidentiary purposes, law enforcement officials must have all conversations related to a particular person or case originally recorded on the same machine. For example, a first call of a target mobile unit is monitored by receiver A, the receiver that was monitoring the forward control channel when the target's call was established. Sometime after terminating that call, the target makes or receives another call, but this time, because the target has moved, receiver B is monitoring the forward control channel for the target's new location and therefore receiver B monitors the call. Since receivers A and B have separate recorders, these two intercepted calls are recorded on-different recorders.
Another prior art system, shown in FIG. 5, allows simultaneous monitoring of multiple targets by a single system, having a plurality of receivers and a baseband switching matrix. The receivers are designated with reference numerals 41A, 41B, 41C, 41D, 41E, and 41F in FIG. 5. A switching matrix 42 provides a path to route the audio signal from each receiver over independent audio signal lines 43 to a separate dedicated monitoring station, identified with reference characters 44A, 44B, and 44C. Control signals from a computer 45 via a controller 46 are input to the switching matrix 42. Control signals are also sent to each receiver from a controller 46 over control lines 47. The computer 45 controls the switching matrix 42 and the receivers 41A through 41F. The capabilities of this system are limited by the current configuration for the computer 45, which usually include no more than six serial ports, with a single port required to control each receiver. Thus only six such receivers can be controlled by the computer 45. Also, the receivers 41A through 41F and the computer 45 must be collocated to reduce signal deterioration due to line length over the control lines 47.
In this FIG. 5 prior art system the audio signal from any receiver can be routed via the switching matrix 42 to any one of separate dedicated monitoring stations 44A through 44C. In practice, the switching matrix 42 is controlled so that the audio from the receiver that identifies the target mobile unit is recorded on a designated monitoring station and then all subsequent conversations that are also related to that target are also recorded on the same monitoring station. In this way the intercept system provides a single tape on which is located all conversations that are relevant to a specific case, where a case is all the telephone conversations relevant to a single wiretap authorization. The disadvantage, however, is that hardware limitations associated with the switching matrix 42 limit the number of recording apparatuses to approximately three. If the system is tracking more than three targets or cases, there will be multiple case recordings on at least one of the recorders. In practical implementation, the switching matrix 42 and its control are highly complex as any of the receivers 41A through 41F may at anytime be switched to any of the monitoring stations 44A through 44C. The complexity of this prior art system grows exponentially more cumbersome with the addition of each receiver and monitoring station.